Hope you are well today and thanks for reaching out to ‘Responsive Menu Pro’ support.

I read your query carefully and here is my response regarding the same.

In the ‘Page Overlay’ section, when you enable the Overlay option. You can set the background overlay colour as per your requirements in the ‘colour’ field. You don’t need to write ‘custom css’ for this simple task.

‘Color’ field takes rgba() color values. An rgba() color value is specified with: rgba(red, green, blue, alpha). First 3 parameters define the intensity of colour and ranges between 0-255. The last value defines the opacity of colour and ranges between 0.0(fully transparent) – 1.0(fully opaque).